Process for producing dry lime hydrate



United States PatentO No Drawing. Continuation of application Ser. No.

850,079, Nov. 2, 1959. This application Apr. 22, 1963, Ser. No. 274,8119 Claims. (Cl. 23-188) The invention relates to a new and useful methodof making commercial hydrated lime. More particularly, the invention isdirected to an improved method of making lime hydrate from afast-acting. quicklime." The term fast-acting quicklime as used in thisspecification and claims means a quicklime which undergoes rapidhydration when contacted withwater, the hydration or slaking time beinggenerally below about one minute, and in many instances of the order ofseconds or less. The present application is a continuation ofapplication Serial No. 850,079, filed November 2, 1959, now abandoned.

One of the essential properties of hydrated lime for use as mortars,plasters, etc., is good plasticity. Plasticity, as it refers to hydratedlime may be defined as a property Which renders the hydrated limecapable of spreading easily on an absorbent surface, i.e. plasticitydepends directly upon the ability of the material in putty form to holdits water against the suction of an absorbent surface to which it isapplied.

In order that hydrated lime may be classified as a plastic finishinglime, it must so react with water as to produce a putty having aplasticity of 200 or greater as determined by the Emley plasticimeterdeveloped by the United States Bureau of Standards.

Certain requirements are essential to the proper hydration of quicklimein order for the hydrate to have the desired degree of plasticity. Tobegin with, a large excess of water must be present at the time ofhydration. By this is meant not that a large excess of water over thattheoretically needed must be present at the start of the hydration ofthe .mass of quicklime, but that there must be water in the liquid phaseavailable to practically every particle of quicklime at the time thatthe particle of quick lime changes from the oxide form to the hydroxideform. It is well known that calcium oxide hydrated essentially in thepresence of steam is extremely nonplastic.

With fast-aoting quicklimes, hydration appears to take place at thesurface of the particles, and the heat of hydration is so great that thewater is converted to steam before it can penetrate the particles. Infact, with some very fast-acting quicklimes, the hydration reaction maybe explosive in nature. Thus, the greater portion of the quicklime ishydrated in an atmosphere of steam rather than being surrounded by waterin the liquid phase.

. This is true even if considerably more water is used than is necessaryto satisfy the chemical requirementof the quicklime.

Attempts have been made to overcome this condition i by e'ndeavoring toprevent the temperature of the water 3,106,453 Patented Oct. 8, 1963 iceunder pressure in the presence of water in the liquid state for asuflicient time to hydrate the calcium content and the major portion ofthe magnesia content of the quicklime. When the hydration has beencompleted, the resulting wet slaked quicklime is dried by eflfecting afinely divided dispersive discharge therefrom tinto an atmospherecapable of vaporizing substantially spontaneously the excess watercarried by the lime particles dicharged, while still in a dispersedstate, leaving a substantially dry powdered hydrated lime. If theprocess according to these patents is of the continuous type, a slurryof quicklime and water is continuously introduced to the hydrationcylinder and the hydrated lime, water and steam are continuouslydischarged.

In processes for hydrating lime, such as described in theabove-mentioned patents, wherein water is mixed with the lime and themixture is conveyed to the hydration cylinder, fast-acting limes whichhave an essentially instantaneous hydration time, e.g. a hydration timeof about 5 seconds or less, are extremely difficult to handle. Theselimes, upon addition of water thereto, are converted almost immediatelyinto a thick paste. In conveying the mixture of lime and water to thehydrator, as for example by pumping, the mixture will solidify due tothe hydration reaction, and the flow of the mixture to the hydrator isdisrupted.

In order to overcome this problem, resort has been made to increasingthe slaking time of the quicklime. For example, resort has been made tocalcining of the lime at higher temperatures and/ or for longer periodsof time [than ordinarily used has been to etfect an increase in slakingtime, but this procedure results in a reduction in plasticity andadversely affects other properties of the ultimate hydrated lime. Thehigh temperatures and/ or long burning times employed result incombination of silicon, iron, aluminum and other impurities in the limein the form of undesirable ferrites, silicates, etc., the presence ofwhich adversely affects the desired physical properties of the hydratedlime, such as plasticity, strength, sand carrying capacity, etc. Inaddition, hard burning involves increased production costs.

With these observations in mind a primary object of the presentinvention is to provide an improved method for making lime hydrate.

Another object of this invention is an improved method for making ahydrated lime from a fast-acting quicklime involving increasing theslaking time of the quicklime without adversely afiecting the desirablephysical properties of the resulting lime hydrate.

A further object if this invention is the provision of an improvedmethod of making lime hydrate of substantially increased plasticity.

Still another object of this invention is a method, for making afinishing lime from a quicklime normally unsuitable for such purpose.

A still further object of this invention is the provision of a methodfor treating a quicklime, particularly a fastacting quicklime, to renderit in a condition for hydration so as to provide a lime hydrate of highplasticity.

Yet another object of this invention is the provision of a method forincreasing the plasticity of a hydrated fast-acting quicklime withoutresort to the use of pressure hydrating equipment.

Still another object of the invention is a process for obtaining higherplasticities for hydrated fast-acting quicklimes regardless of whetherhydration is carried out under atmospheric or superatmospheric pressure.

These and other objects of the invention will become more clearlyapparent from this specification and appended claims.

The process according to this invention comprises bringing a gaseousreactant selected from the group consisting of carbon dioxide, watervapor, and mixtures thereof, into contact with particles comprising afast-acting quicklime at a temperature at which the gaseous reactantcombines therewith, to form on the surface of the particles a filmcomprising in combination the gaseous reactant and quicklime, theremainder of said particles consisting essentially of quicklime, andthereafter hydrating the quicklime to form a substantially dry, powderylime hydrate.

The process of this invention produces a number of extremely desirableresults. One of the most important advantages to be obtained thereby ishydrated limes of substantially increased plasticity. As a matter offact, by the present method quicklimes heretofore considered unfit foruse as plastic finishing limes when in the hydrate form, because oftheir extremely low plasticity, may be converted into hydrates having asufficiently high plasticity, about 200 or greater, to be useful asmortars, plasters, etc. This result is particularly surprising in viewof the fact that if quicklime is hydrated in an atmosphere of steam, theresulting product is extremely non-plastic, and that if recarbonationtakes place during the calcination of limestone, the resultingquicklime, when hydrated, is also extremely non-plastic.

A further benefit to be obtained by the method of this invention is anincrease in the slaking time of fast-acting quicklimes without areduction in plasticity. By increase in slaking true as used in thespecification is meant an increase in the time interval between initialcontact of the quicklime with water and the time at which the hydrationreaction has progressed sulficiently to produce steam. This result alsois surprising in view of the fact that reduction in plasticity appearsto accompany any increase in slaking time effected by methods heretoforeknown. The increase in slaking time is particularly advantageous inthose processes where the lime is mixed with water and subsequentlyconveyed to the hydration cylinder as in the processes described inUnited States Patents Nos. 2,309,168 and 2,356,736, briefly discussedabove. This increase in slaking time permits transfer of the quicklimewater mixture from the point where the quicklime and water are combinedto the hydration cylinder without fear of the mixture forming anon-flowable, thick mass.

it is believed that by the method of this invention the quicklimeparticles become coated at least in part with a thin film, possibly alayer only a few molecules thick, comprising in combination the gaseousreactant and quicklime. The surface of the particles upon which thisfilm is formed comprises, in addition to the outer shell of theparticles, interior surface portions provided by the porous latticestructure of the quicklime particles which are formed as a result ofcalcination of the limestone. The exact manner in which the gaseousreactants combine with the quicklime particles is not entirelyunderstood. The molecules of reactant may be merely adsorbed as such onthe surface of the particles, or they may react chemically with thequicklime to form a surface film of calcium carbonate or calciumhydroxide, or both, depending upon the particular gaseous reactant orreactants employed. Thus the term combine as used in this specificationand claims means any physical or chemical association of the gaseousreactants with the surface of the quicklime particles.

This film comprising in combination the reactant and quicklimeapparently permits water in the liquid phase to penetrate the particlesof the quicklime, the interior of which consists essentially ofquicklime, so that throughout the interior of each particle, water inthe liquid phase is present when the quicklime changes from the oxideform to the hydroxide form, a condition believed to contributematerially to the high plasticities obtained by the method of thisinvention.

The fast-acting quicklimes which may be employed in the method or" thisinvention comprise any of the well known types of quicklimes, such ashigh calcium quicklimes whose magnesium content generally runsapproximately 2% or less but it may run as high as from 540%; highmagnesium quicklimcs which are usually made from practically puredolomite and contain approximately 40% magnesium oxide; and quicklimesin between those two magnesium contents which are usually known asmagnesium quicklimcs.

Because of the high porosity of quicklime particles, the method of thisinvention may be satisfactorily carried out with a wide range of sizesof particles. The particles may be as small as those found in fine dustsup to large lump sizes. Preferably, the particles are ground so that thelarger particles do not substantially exceed about /1 inch.

The quicklime particles should be at a temperature at which thereactants combine rather readily therewith. The temperature willprimarily depend upon the reactant gas employed.

In the method of this invention where the reactant gas is carbondioxide, the particles of quicklime should be at a temperature in therange between about 400 F. and below the decomposition temperature ofthe limestone, which will run between about 1400 F. and l650 R,depending upon the chemical composition of the limestone. Within thisrange of temperatures carbon dioxide combines readily with calciumoxide, particularly if traces of water vapor are present. Below about400 F. there is little combination of calcium oxide with carbon dioxideunless moisture is present. If carbon dioxide is the sole reactantemployed, the quicklime particles are preferably at a temperature withinthe range between about 500 F. and 800 F.

Where the reactant gas consists essentially of carbon dioxide, andmoisture is present either in the quicklime being treated or in gaseousstate, as would be the case in a stack gas, the combining of thereactant with quicklime proceeds even at room temperature.

If water vapor is the primary reactant the temperature of the quicklimeparticles may be at room temperatures or at elevated temperatures,preferably above the boiling point of water. The preferred range oftemperatures where the principal reactant comprises water vapor isbetween about 250 to about 450 F.

The temperature of the quicklime particles where the reactant gascomprises a mixture of carbon dioxide and water vapor, such as thatexemplified by a stack gas, is generally the same as that employed wherethe reactant is water vapor.

As stated above, the gaseous reactants employed in this inventioncomprise carbon dioxide and water in the gaseous state, eg steam. Thesegaseous reactants may be used alone, in combination with each other, orin combination with other gases which do not react to any measurabledegree with quicklime so as to adversely affect the plasticity of theultimate lime hydrate. For example, the reactant gas may comprise purecarbon dioxide or steam or mixtures of these two gases. Also, thereactant gases may be present as constituents in flue gases resultingfrom the combustion of solid, liquid or gaseous fuels. In this case thequicklime particles may be contacted directly with a flue gas;preferably the flue gas is at elevated temperatures. A particularlyuseful gas containing both of the gaseous reactants is the flue gas froma lime kiln, which gas contains substantial portions of both water vaporand carbon dioxide and is readily available to the manufacturer ofhydrated lime. Air, particularly moist air may be used.

Quicklimes vary enormously in both physical and chemical propertiesdepending upon the limestone source and the calcination procedure usedin making the quicklime. To a large degree the amount of gaseousreactant required for obtaining the highest plasticities will dependupon the particular quicklime composition and the particular gaseousreactant employed. However, as a general rule the benefits of thisinvention can be obtained by using suffiutes in others.

cient water vapor so that from about 0.1 to about 5 '10-- molsthereofcombine with the quicklime per square meter of surface area ofthe quicklime. Preferably, sufficient water vapor is used so that fromabout 0.5 X 10 to about 2 10- mols of water vapor are, combined persquare meter of surface area of the quicklime. This amount .of watervapor ordinarily will combine with dioxide and water vapor, as would bepresent in a flue gas,

consideration should be given to the relative proportions of the tworeactants. In most instances a few simple laboratory tests, similar tothose of the following specific examples, will readily disclose to theuser of this invention the amounts of the particular gaseous reactantrequired to provide a given quicklime with the highest plasticities.

The gaseous reactants should be at somewhat elevated temperatures whenbrought in contact with the quicklime particles. If the reactant iswater, the temperatures should be sufliciently high to insure that thewater is in the gaseous state, e.g. above 212 F. at atmosphericpressure. The temperature of the reactants ordinarily should be withinthe range between about 212 F. and 1000 F., although temperatures ashigh as 1500 'l650 F. may be employed. 'No particular advantage is to begained by temperatures substantially above 1000 F. The preferred rangeof temperatures for the gaseous reactants is from about, 250 to about800 F.

After the quicklime particles have been contacted with the reactant gasas described above, the quicklime may be hydrated by any of the wellknown hydration processes. For example, the treated quicklime maybehydrated in such well knownghydrators as'the Corson, the Kritzer, theSchaffer, theSchulthess, or the Clyde hydrators. If the quicklime hasbeen treated under optimum conditions, increased plasticities willresult regardless of the method of hydration. i i

According to the preferred form of this invention'the treatedquicklimeis hydrated according to the method of the United Statespatents discussed hereinabove, and United States Patent No. 2,409,546,wherein clustering of the finely divided hydrate in powder form afterpressure hydration is disclosed. it By combining the method of thisinvention with the hydration process of these patents even higherplasticities are obtained.

The plasticities referred to hereinabove and in the following examplesare plasticities available substantially immediately with water asdistinguished from a plasticity requiring hours of soaking to develop.

The following examples are illustrative of the present invention.

Example I The apparatus employed comprised a chamber with a lineconnected to the top thereof for delivering steam to the chamber. Thebottom of the chamber was provided with a screened vent. Steam at 140 F.was introduced to the chamber which contained about 300 grams of anAustralian High calcium quicklime (approximately 83.5% CaO; 3.5% MgO;8.9% silica as SiO and 1.8% Fe O having a particle size of about 97%through a mesh screen, 69% through a 50 mesh screen, through a 100 meshscreen, and 2.5% through a 200 mesh screen. The steam was introduced atthe rate of about 8.8 ml. per minute for a period of 18 seconds.

300 g. of quicklime prepared as above was placed in a stainless steelbeaker and to the quicklime was added 100 g. of water, and the water andquicklime were intimately mixed. This amount of water was determined asthat amount theoretically required to completely hydrate the quicklime.Immediately upon addition of water to the quicklime, the stainless steelbeaker was placed in an autoclave containing water in the bottom thereofand sealed from the atmosphere. The water in thebottom of the autoclavedoes not come in contact with the quicklime, and is converted to steamby electric heaters to build up pressure in the autoclave.

The pressure in the autoclave increased to about pounds per square inchin about 15 minutes, and these conditions were maintained forapproximately 20 minutes. At the end of the 20 minute period thepressure in the autoclave was reduced to atmospheric pressure insteam-treated quicklime was increased from about 5 seconds to about 15seconds.

Example II Approximately 300 g. of the lime of Example I was treatedwith Bunsen burner gas in the apparatus of that example for a period ofone minute. Approximately 600 g. of quicklime thus prepared was placedin a hydration cylinder which is provided with a mechanically drivenagitator. The cylinder is also provided with a line for delivering waterto the cylinder under pressure, and :a discharge valve for dischargingthe contents of the cylinder into a porous, bag-type container throughwhich steam and other gases may escape.

The cylinder, which was at a temperature of about 250 F., was sealedfrom the atmosphere and about 433 g. of water were introduced to thecylinder under pressure. This amount of water is 120% in excess of theamount of water necessary to effect complete hydration of the lime.

Upon addition of water to the cylinder, the pressure in the cylinderincreased, and after approximately 15 seconds the pressure was about 225p.s.i. At the end of about 20 minutes the pressure dropped to about 60p.s.i. and the discharge valve Was opened whereby substantially theentire contents of the cylinder, comprising hydrated lime, water andsteam was discharged into the porous collecting bag. Due to theintrinsic heat of these materials the water was flashed from thehydrated lim leaving a dry powdery product.

Plasticities for the hydrated lime so produced were obtained by means ofan Emley plasticime-ter, and the plasticity was found to be 286. Ahydrated lime produced from the :same quicklime raw material, but whichhad not been treated with carbon dioxide had a plasticity of 149. Theslaking time of the treated quicklime was about 15 seconds as comparedto about 5 seconds for the untreated quicklime.

Example 111 Approximately 300 g. of a Pennsylvania high calcium lime(92% C210; 6% MgO; 1% silica; 1% Fe O and other oxides) were placed inthe apparatus of Example I. The sieve analysis for this lime was 97.2%through mesh sieve and 1.6% through 200 mesh sieve. Carbon dioxide atroom temperature was introduced to the chamber at a rate providing apressure of 10 inch column of water for a period of 3 minutes.

300 g. of quicklime prepared as above was placed in a stainless steelbeaker with 100 grams of water, which amount of water is thattheoretically required to completely hydrate the quicklime. The samplewas autoclaved in the same manner as in Example I.

Plasticity was obtained by means of an Emley plasticirncter and theplasticity of the hydrated lime produced according to this example was163. Hydrated lime produced from the same quicklime raw material andhydrated in the same manner, but which had not been subjected to thetreatment with carbon dioxide, had a plasticity of 122. The slaking timeof the treated quicklime was about 12 seconds as compared to about 6seconds for the untreated quicklime.

Example I V Approximately 300 g. of a Canadian dolomitic quicklime(56.7% CaO; 42.2% MgO) having a screen analysis, 97.7% through a 20 meshscreen and 73.5% through a 200 mesh screen, was treated with cool carbondioxide gas in the apparatus of Example I for a period of seconds. Therate of how of carbon dioxide gas to the sample was that equivalent toinch column of water.

200 g. of quicklime prepared as above was mixed with 66.6 ml. of waterin a Hobart mixer and the plasticity was determined by an Emleyplasticimeter. The plasticity was 253. Hydrated lime produced from thesame quicklime raw material and hydrated in the same manner, but whichhad not been subjected to the treatment with carbon dioxide, had aplasticity of 203. The slaking time of the treated quicklime was 95seconds as compared to 22 seconds for the untreated quicklime.

What is claimed is:

1. A method for producing dry, powdery hydrated lime which comprisespreparing a fast-acting quicklime for hydration by bringing a gaseousreactant selected from the group consisting of water vapor and mixturesof carbon dioxide and water vapor into contact with a particulatefast-acting quicklime having a hydration time below about one minute, ata temperature to form on the surface of said particles a dry filmcomprising in combination said gaseous reactant and said quicklime, saidtemperature and the time of contact between said gaseous reactant andsaid quicklime particles being such that not more than about 5 l0 molsof water vapor and not more than about 2 l0- mols of carbon dioxidecombine with said quicklime for each square meter of surface area ofsaid quicklime, the unreacted portion of said particles consistingessentially of dry quicklime, whereby the slaking time of said quicklimeis increased by at least about 100%, and thereafter hydrating saidparticles of quicklime with water essentially in the liquid state toform a substantially dry, powdery lime hydrate.

2. The method according to claim 1 in which said gaseous reactantcomprises a flue gas containing carbon dioxide and water vapor.

3. The method according to claim 2 in which the quicklime is at atemperature within the range between about 250 and about 450 F., and theline gas is at a temperature within the range between about 212 andabout 1000 F.

4. In the method of making a dry, powdery lime hydrate in which thecalcium oxide content and at least a major portion of the magnesiacontent of a fast-acting quicklime are hydrated under pressure withWater in excess of that which will combine as water of hydration and theresulting slaked quicklime is dried by removing surplus water from thefinely-divided hydrated lime particles by releasing the pressure toprovide a powdery mass in which the particles of said hydrated lime aremaintained in a finely-divided condition, the step which comprises priorto hydration preparing said fast-acting quicklime for hydration bybringing a gaseous reactant selected from the group consisting of .watervapor and mixtures of carbon dioxide and water vapor into contact with aparticulate fast-acting quicklime having a hydration time below aboutone minute, at a temperature to form on the surface of said particles adry film comprising in combination said gaseous reactant and saidquicklime, said emperature and the time of contact between said gaseousreactant and said quicklime particles being such that not more thanabout 5X10 mols of water vapor and not more than about 2 10- mols ofcarbon dioxide combine with said quicklime for each square meter ofsurface area of said quicklime, the unreacted portion of said particlesconsisting essentially of dry quicklime, whereby the slaking time ofsaid quicklime is increased at least by about percent.

5. The method according to claim 4 in which said gaseous reactantcomprises a line gas containing carbon dioxide and water vapor.

6. The method according to claim 5 in which the quicklime is at atemperature within the range between about 250 and about 450 F., and theflue gas is at a temperature within the range between about 212 andabout 1000 F.

7. In a method for producing a dry, powdery hydrated lime in which thecalcium oxide content and at least a major portion of the magnesiacontent of a fast-acting quicklime are hydrated under pressure withwater in excess of that which will combine as water of hydration, theresulting slaked quicklime is dried by removing surplus vuater from thefinely-divided hydrated lime particles by releasing the pressure toprovide a powdery mass in which the particles of said hydrated lime aremaintained in a finely-divided condition, and the finely-divided hydratein powder form is subsequently clustered by pounding the hydrate as apowder between surfaces, a step which comprises prior to hydrationpreparing said fastacting quicklime for hydration by bringing a gaseousreact-ant selected from the group consisting of water vapor and mixturesof carbon dioxide and water vapor into contact with a particulatefiast-acting quicklime having a hydrating time below about one minute,at a temperature to form on the surface of the particles a dry filmcomprising in combination said gaseous reactant and said quicklime, saidtemperature and the time of contact between said gaseous reactant andsaid quicklime particles being such that not more than about 5 l0- molsof Water vapor and not more than about 2X10 mols of carbon dioxidecombine with said quicklime for each square meter of surface area ofsaid quicklime, the unreacted portion of said particles consistingessentially of dry quicklime, whereby the slaking time of said quicklimeparticles is increased by about 100 percent.

8. The method according to claim 7 in which said gaseous reactantcomprises a flue gas containing carbon dioxide and water vapor.

9. The method according to claim 8 in which the quicklime is at atemperature within the range between about 250 and about 450 F., and theline gas is at a temperature within the range between about 212 andabout 1000 F.

No references cited.

1. A METHOD FOR PRODUCING DRY, POWDERY HYDRATED LIME WHICH COMPRISESPREPARING A FAST-ACTING QUICKLINE FOR HYDRATION BY BRINGING A GASEOUSREACTANT SELECTED FROM THE GROUP CONSISTING OF WATER VAPRO AND MIXTURESOF CARBON DIOXIDE AND WATER VAPOR INTO CONTACT WITH A PARTICULATEFAST-ACTING QUICKLINE HAVING A HYDRATION TIME BELOW ABOUT ONE MINUTE, ATA TEMPERATURE TO FORM ON THE SURFAE OF SAID PARTICLES A DRY FILMCOMPRISING IN COMBINATION SAID GASEOUS REACTANT AND SAID QUICKLINE, SAIDTEMPERATURE AND THE TIME OF CONTACT BETWEEN SAID GASEOUS REACTANT ANDSAID QUICKLINE PARTICLES BEING SUCH THAT NOT MORE THAN ABOUT 5X10**-4MOLS OF WATER VAPOR AND NOT MORE THAN ABOUT 2X10**-4 MOLS OF CARBONDIOXIDE COMBINE WITH SAID QUICKLINE, THE UNREACTED PORTION OF SAIDPARTICLES CONSISTING ESSENTIALLY OF DRY QUICKLIME, WHEREBY THE SLAKINGTIME OF SAID QUICKLIME IS INCREASED BY AT LEAST ABOUT 100%, ANDTHEREAFTER HYDRATING SAID PARTICLES OF QUICKLIME WITH WATER ESSENTIALLYIN THE LQUID STATE TO FORM A SUBSTANTIALLY DRY, POWDERY LIME HYDRATE.